1. Field of the Invention
The invention relates to side-emitting step index fibers, methods for producing such, and their applications.
2. Description of Related Art
Step index fibers are understood as light-guiding fibers, the light being guided in the fiber core by total reflection of the light guided in the core at the cladding enclosing the fiber core along the fiber axis. The total reflection occurs whenever the cladding has a lower refractive index than the fiber core guiding the light. However, the condition of total reflection is possible only up to a limiting angle of the light striking the cladding, which angle is a function of the refractive indices of core and cladding. The limiting angle βMin, that is to say the smallest angle at which total reflection still occurs, can be calculated by sin(βMin)=n2/n1, with βMin being measured from a plane perpendicular to the fiber axis, n1 representing the refractive index of the fiber core, and n2 representing the refractive index of the cladding.
In general, the goal is to guide the light in the fiber as well as possible, that is to say as little light as possible is to be lost during coupling into the fiber and during transport in the fiber. The side-emitting step index fiber is a step index fiber in the case of which light is intentionally coupled out of the fiber core and of the fiber. What is desired in general is a uniform coupling out that, in the ideal case, makes a side-emitting step index fiber appear as a uniformly, luminous band or line. This renders it of interest for multiple applications, particularly in lighting engineering.
In the sense of the invention, side-emitting means that the fiber is capable of emitting light laterally irrespective of whether it is in operation, that is to say whether a light source is actually connected and the light is switched on.
As is generally known, the fibers are produced with the aid of fiber drawing processes in which at least the preform of the fiber core is heated up to or in excess of the softening temperature of material of the preform or the fiber core, and a fiber is drawn out. The principles of the fiber drawing process are described in detail in, for example, German patents DE 103 44 205 B4 and DE 103 44 207 B3.
Various methods are known from the prior art for producing the effect of side emission. One known method is to ensure coupling out of light in the fiber core.
Japanese laid-open application JP 9258028 A2 discloses side-emitting step index fibers in the case of which the light is to be coupled out by a non-round core. The coupling out is performed when light strikes the interface between fiber core and cladding at angles that are smaller than the limiting angle of total reflection βMin. The non-round core geometries described, for example square, triangular or star-shaped, produce in the core geometric regions in which light otherwise guided by total reflection can be coupled out. The production of side-emitting fibers via such core geometries is, however, attended by the problem that coupling out the light is very inefficient in this case. The light is guided in the fiber to the cladding at essentially very flat incident angles, and the described core geometries extend along the fiber axis. Consequently, there are scarcely any areas where βMin is undershot. Furthermore, making use of the core geometries disclosed in JP 9258028 A2 for fibers made from glass is very complex because it is very difficult to produce appropriate preforms such as are required for fiber drawing. Moreover, precisely in the case of glass fibers the ultimate strength of such fibers is greatly reduced with non-round fiber core diameters. Presumably for this reason, this publication also discloses only fibers made from polymers.
A further method for coupling light out of the fiber core is described in U.S. Pat. No. 4,466,697, according to which particles that reflect and/or scatter light are mixed in the fiber core. There are difficulties here in producing relatively long fibers with uniformly side-emitting properties, since the guiding of light in the core is reduced by absorption by the added particles in the core, since there are no particles that scatter completely, but only ones that scatter nearly all the incident light. Because in the case of particles distributed uniformly in the core there is a very high probability that the light guided in the core will strike such particles, the probability of absorption is also very high even when the total number of particles is small. The coupling-out effect can therefore be scaled only with great difficulty, and this renders reproducible results in fiber drawing a matter of extreme complexity extending as far as near impossibility at least for fibers over 3 m in length, at least when the aim is to produce glass fibers.
Scalability in the sense of the present disclosure is understood as the capacity for targeted setting of the side-emission effects over the length of the fiber. This is required because fiber lengths can vary very strongly for different applications, whereas the aim is to attain the most uniform intensity possible for the emission of light over the entire fiber length.
As an alternative to coupling the light directly out of the fiber core, side-emitting properties can also be caused in the case of fibers by effects in the interface between fiber core and cladding or in the cladding itself. Thus, it is known from the prior art that crystallization reactions between core glasses and cladding glasses are undesirable, since the crystallites in the interface between core and cladding can serve as scattering centers such that light is coupled out of the fiber and therefore reduces its optical conductivity. This effect is generally undesirable in the case of optical waveguides and, as described in German patent DE 102 45 987 B3, glass fibers are normally developed in a targeted way such that no crystallization takes place between core and cladding. However, it would be conceivable to make targeted use of crystallization between core and cladding so as to produce side-emitting properties. Crystallization occurs during fiber drawing when core and cladding fuse with one another and the fiber cools down again. However, it has emerged in experiments that the crystallization process can be set and controlled during fiber drawing only with difficulty, and so there has so far been no success commercially with a reproducible and scalable production of side-emitting glass fibers whose side-emitting properties are based on the presence of crystallites in the interface between core and cladding.
In order to produce side-emitting properties based on scattering centers in the interface between core and cladding, it is proposed in accordance with patent specification LV 11644 B for silica glass fibers to apply to the drawn out silica glass fiber a coating that contains scattering particles. The outer protective cladding around the silica glass fiber can be applied subsequently. As is customary with silica glass fibers, the coatings both of the scattering layer and of the outer cladding consist of plastics. This has the disadvantage that the drawn out fiber core has to be subjected to further coating steps and is unprotected throughout this. Dirt particles deposited between core and coating lead to possible breaks and/or to points with strong coupling out of light. In any case, silica glass fibers as such are already extremely expensive because of the material, but the complicated fabrication method required in this publication renders them yet more expensive in addition.
US 2005/0074216 A1 discloses a side-emitting fiber having a transparent core made from plastic that firstly has a transparent first cladding and there after a second cladding, the two likewise being made from plastic. Scattering particles are embedded in the second cladding, which is the outer cladding. This method is possible only for fibers with very large fiber diameters of 4 mm or more, because the light guided in the fiber core must be coupled out by the inhomogeneities necessarily present at the very large interface between core and first cladding. The second cladding with the embedded scattering particles serves in this case to homogenize the coupled-out light over all solid angles. However, fibers having such large core diameters are less flexible and can therefore be laid only with difficulty. Such embodiments can be produced from glass only as rigid light-guiding rods and are completely inflexible.
A severe disadvantage of all solutions described that include plastic is, furthermore, that all of the plastic claddings described are flammable. Consequently, such fibers should be generally undesirable. Apart from this, they cannot be allowed at least in areas having more stringent fire protection regulations for example inside aircraft cabins.
Glass fibers as such are not flammable. Side-emitting glass fibers are, however, likewise already known. The established method for producing glass fibers having side-emitting properties provides for the preform of the fiber core to be roughened by grinding or sandblasting. These treatment processes produce structures on the peripheral surface of the fiber core that project into the fiber core and are intended to couple out the guided light. It has emerged here, as well, that the process for producing the side emission is inefficient and also scalable only with difficulty. Moreover, the treatment of preforms, in particular when these consist of glass, is often expensive and complicated. Moreover, the structures projecting into the fiber core constitute instances of damage to the fiber core that can give rise to load peaks and therefore to cracks in the event of bending loads, the result being that such fibers suffer from a reduced ultimate strength. For this reason, as well, this technique seems in need of improvement.